The invention relates to compensation of harmonics of an electrical network, and particularly to compensation to be carried out by a plurality of units in a distributed manner.
A great number of domestic and industrial electrical devices derive their current from an electrical network, the curve form of the current differing from the ideal, i.e. sinusoidally alternating, curve form. While deriving their current from the electrical network, such devices cause harmonics, i.e. harmonic components, to occur in the electrical network. Often the harmonics generated by the devices are formed at frequencies of certain multiples of the basic frequency of the electrical network. If the magnitude of the harmonics increases in a distribution network, the quality of electricity becomes so poor that the electric devices do not necessarily work or they constitute a risk factor to personal and device security.
Examples of devices generating harmonics include rectifiers and various choppers. A simple diode rectifier generates direct voltage from a supplying voltage and derives current from the network only when the voltage of the network is higher than the rectified voltage. Typically, current is derived from the network only at voltage peaks of the supplying network, in which case the current is pulse-like, thus generating harmonics to the network.
In distribution networks, typically, an aim has been to compensate the harmonic components of the network by using passive filters to form absorbing circuits for certain frequencies at which harmonic components occur.
In an industrial environment in particular, attempts have been made to compensate network harmonics by using active network bridges particularly designed for this purpose to measure the harmonics contents of the network and, on the basis of the measurement, supply the network with a current which compensates the measured harmonics of the network. The usage of an active bridge as a separate electrotechnical device for compensation is an expensive solution, and the compensation capacity of the device is limited. As the magnitudes of the harmonic components to be compensated increase, a plurality of such devices has to be coupled to the same network. Each compensation device constitutes an independent unit of its own, so it is a challenging task to implement compensation by a plurality of such parallel devices.